The evolution of wireless communication over the past century, since Guglielmo Marconi's 1897 demonstration of radio's ability to provide continuous contact with ships sailing the English Channel, has been remarkable. Since Marconi's discovery, new wireline and wireless communication methods, services and standards have been adopted by people throughout the world. This evolution has been accelerating, particularly over the last ten years, during which the mobile radio communications industry has grown by orders of magnitude, fueled by numerous technological advances that have made portable radio equipment smaller, cheaper and more reliable. The exponential growth of mobile telephony will continue in the coming decades as well, as this wireless network interacts with and eventually overtakes the existing wireline networks.
Pursuant to a recent Federal Communications Commission (FCC) Ruling and Order, cellular phone service providers within the United States must provide by October 2001 the capability to locate the position of a cellular phone making an emergency (911) call within the provider's system to within 125 meters with about 67% probability, i.e., within one standard statistical deviation. A variety of techniques are currently under study. All measure the cellular phone's position by reference to establish points whose positions are known. The phone's location is estimated to lie at the best intersections (n the least-squares or maximum likelihood sense, since all location measurements have random errors) of curves whose shapes and locations are determined by the particular technique utilized, as described in more detail hereinafter.
The Global Positioning System (GPS), which uses several satellites as reference points, generally provides the requisite high degree of accuracy in location measurements, well within the FCC's constraints. GPS systems, however, although providing accurate measurements under favorable conditions, do not function well inside buildings or other areas obscuring access to the orbiting satellites, e.g., "urban canyons" formed by multiple, adjacent high-rise buildings such as found in any large city. Furthermore, GPS is not compatible with existing cellular phones and the incorporation of GPS technology into a cell phone or other mobile terminal would require extensive hardware additions and modifications.
Another technique, signal strength measurements, establishes circular arcs around the base stations servicing the mobile terminals. This technique, however, is hampered by a poor understanding of path loss, and hence the errors are large. Angle of Arrival (AOA) measurements intersect radii from the base stations to pinpoint location. As few as two base stations suffice to determine an intersection unless the mobile terminal is near to the line between the base stations. This, however, requires expensive modifications to base station antennas, which are further complicated by neighborhood resistance to obtrusive antennas.
A technique known as Time Difference of Arrival (TDOA) requires that the base stations be synchronized with respect to each other. This may be done using inexpensive GPS receivers at the base stations, or by other techniques. In TDOA one measures the time differences between the arrivals of a given signal from the mobile terminal at three or more base stations. This does not require any timing information from the mobile phone itself. As will be discussed further herein, two base stations together will determine a hyperbola; the mobile terminal will lie upon one of the branches of the hyperbola. The sign of the TDOA determines the particular hyperbolic branch unless the TDOA is within errors of being zero. Three base stations will determine three hyperbolic curves, of which two contain independent information, and the mobile terminal will lie at the intersections of respective branches of those curves.
Another technique takes advantage of the Timing Advance (TA) which is used in all Time Division Multiple Access (TDMA) systems to determine the amount of timing the mobile terminal broadcast burst must be advanced in order to fit into the proper time slot. An advance is required if the mobile terminal is sufficiently distant from the base station that its burst would otherwise be significantly retarded due to the time of transit to the base station. TA depends on the distance from the serving base station and thus is a measure of the distance from that station. By using various techniques, the TAs from several base stations can be measured, forming a corresponding number of circles about the respective base stations, and the mobile terminal will lie at the intersection of the resultant circles. It should be understood that in this invention any technique under the general heading "Timing Advance" measures the time for a signal to travel from the mobile terminal to the base station, regardless of whether or not the measure is actually used in calculating timing advance.
Of the above techniques, Time Difference of Arrival (TDOA) and Timing Advance (TA) enable Automatic Location Identification (ALI) with the use of currently-available mobile terminals with relatively inexpensive modifications to the base station hardware and software. However, TDOA, although practical and under scrutiny in many quarters, is somewhat limited in precision and requires a minimum of three base stations to determine anything more than a single hyperbolic contour along which the target lies. TA, by itself, is even more limited in precision, although only two base stations are adequate to reduce the problem to a two-fold ambiguity within the horizontal plane, within errors, as will be discussed further herein.
In assignee's co-pending patent application, U.S. Ser. No. 08/839,864, entitled "Systems and Methods for Locating Remote Terminals in Radiocommunication Systems," a selective technique is disclosed whereby either TDOA or TA is used to determine the location of a mobile terminal. The co-pending patent application, however, does not disclose or suggest a hybrid technique combining both TDOA and TA into a synergistic system and method as described and set forth in the present application.
In any event, except for GPS, errors inherent within the aforedescribed location measurement approaches do not currently meet the FCC mandate.
It is, therefore, an object of the present invention to reduce the aforementioned inherent errors in measurement for the various techniques.
It is also an object of the present invention to meet the FCC mandate by providing a more accurate system and method to locate mobile terminals within a telecommunications system.
It is a further object of the present invention to provide a system and method for more accurately estimating mobile terminal users implementing emergency signals, whereby emergency response teams may be more accurately guided to and assist the users.